#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,g,s;
	float b,p=0;
	scanf("%d",&n);
	g=n%10;
	s=n/10%10;
	b=n/100%10;
	p=(b+s)/(s+g);
	printf("%.2f",p);
	return 0;
}
/**************************************************************
	Problem: 1608
	User: xiaojingxuan
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2076 kb
****************************************************************/